It is sometimes desirable to vary the partial coherence of the illumination in a microlithographic projection imager. The partial coherence (.sigma.) is a ratio formed of the numerical aperture of the illuminator optical system divided by the numerical aperture of the objective imaging system. ##EQU1## Since the numerical aperture of the objective is generally fixed, the partial coherence ratio can best be varied by changing the numerical aperture of the illuminator optical system. This can be important when a phase shifting mask is used at the reticle plane of the objective imaging system.
The numerical aperture of the illuminator has been varied by changing the size of an aperture stop at the pupil of the illuminator optical system. This adjusts the size of the illumination beam passing through the illuminator pupil; but it blocks part of the illumination in the pupil region and reduces the illumination energy delivered to the objective imaging system, which has the disadvantage of slowing the machine down. The portion of the illumination energy that is blocked by an aperture stop at the illuminator pupil correspondingly reduces the illumination energy delivered to the image plane of the objective imaging system and correspondingly increases the imaging exposure time.
This invention devises a better way of adjusting the numerical aperture of the illuminator optical system, for varying the partial coherence, while substantially preserving the illumination energy so that imaging exposure times are not increased. The inventive way of adjusting partial coherence can be made more convenient than changing an aperture stop at the illuminator pupil and can be arranged to be made at the keyboard of a computer controlling functions of the projection imager.